The location of a device may be determined using a global positioning system (“GPS”). In a general GPS system, a receiver acquires signals from four or more satellite vehicles to obtain a three dimensional location and the current time stamp. A receiver may employ multiple channels and the received signal may be processed in each channel to acquire a signal from a single signal source. After acquisition, a delay-locked loop is traditionally used to track the signal source.
GPS satellite vehicles emit two microwave carrier signals of L1 and L2 frequency. The two microwave carrier signals are modulated by: 1) a C/A code (Coarse Acquisition), 2) a P-Code (Precise), and 3) a data message.
The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) code that modulates the L1 carrier phase. The C/A PRN code comprises 1023 bits in order that are repeated every millisecond. There is a different C/A PRN code for each GPS satellite vehicle. The P-Code modulates both the L1 and L2 carrier phases. The P-Code is a 10 MHz PRN code. The data message modulates the L1-C/A code signal. The data message is 50 Hz signal consisting of data bits that encode a time stamp, the GPS satellite vehicle orbit parameters, clock corrections, and other parameters. All of this data is useful for the receiver to know in order to calculate and update its position.
In one approach, a receiver may attempt to acquire a signal by: 1) generating a replica PRN code emitted by a satellite vehicle that is potentially visible overhead the receiver, and 2) determining a correlation between the received signal and the suitably modulated replica code. Typically, the correlation between the received signal and the replica code is performed by calculating both the In Phase (“I”) and Quadrature (“Q”) correlation integrals. One issue that arises is that the signal from the satellite is also modulated by data bits using phase modulation. These data bits are unknown at a standard standalone receiver and as a result, the I and Q correlation integrals can not reliably be extended coherently beyond the length of one data bit. One existing approach around this problem is to combine the I and Q correlation integrals non-coherently between data bits. This also helps by mitigating the impact of uncertainty in the carrier modulation frequency.
One salient disadvantage to the non-coherent approach is that in instances when the received signal is highly degraded (for example, the receiver that is receiving the received signal is inside a building), then the duration of received signal that needs to be processed to compensate for the level of degradation is roughly proportional to the level of degradation raised to the power of two. To illustrate, suppose the received signal is degraded by an additional factor of 100, then the duration of data needed to compensate for the level of degradation is increased proportional to 1002 if non-coherent processing is done.
Based on the foregoing, there is a clear need for a technique to avoid non-coherent approaches in correlation calculations during signal processing.